The present invention relates to a method and apparatus for ion chromatography using eluents generated within the system.
In liquid chromatography, a sample containing a number of components to be separated is directed through a chromatography separator, typically an ion exchange resin bed. The components are separated on elution from the bed in a solution of eluent. One effective form of liquid chromatography is referred to as ion chromatography. In this known technique, ions to be detected in a sample solution are directed through the separator using an eluent containing an acid or base and thereafter to a suppressor, followed by detection, typically by an electrical conductivity detector. In the suppressor, the electrical conductivity of the electrolyte is suppressed but not that of the separated ions so the latter may be detected by the conductivity detector. This technique is described in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,926,559.
There is a general need for a convenient source of high purity acid or base for use as an eluent for liquid chromatography and, particularly, for ion chromatography. In one technique, described in U.S. Pat. No. 5,045,204, an impure acid or base is purified in an eluent generator while flowing through a source channel along a permselective ion exchange membrane which separates the source channel from a product channel. The membrane allows selective passage of cations or anions. An electrical potential is applied between the source channel and the product channel so that the anions or cations of the acid or base pass from the former to the latter to generate therein a base or acid with electrolytically generated hydroxide ions or hydronium ions, respectively. This system requires an aqueous stream of acid or base as a starting source or reservoir.
There is a particular need in ion chromatography for generating an acid or base internally within an ion exchange bed without the requirement of an aqueous acid or base stream source and for simultaneously suppressing conductivity of the eluent in the ion exchange bed after chromatographic separation.
In copending patent application, Ser. No. 08/781,537, filed Jan. 8, 1997, incorporated herein by reference ("the copending application") method and apparatus are disclosed for generating an acid or base eluent in an aqueous stream solely from an ion exchange bed for liquid chromatography and for simultaneously suppressing conductivity of the eluent in the ion exchange bed after chromatographic separation. The copending application describes a system in which a base is generated for the analysis of anions by ion chromatography. The method uses a bed of cation exchange material (e.g., a resin bed) including exchangeable cations. The bed has first and second bed sections arranged in series. The method includes the following steps:
(a) flowing an aqueous feed stream through a first cation exchange bed section while applying an electrical potential to a cathode to generate hydroxide ions in and assist in displacing some of cations on the bed into the aqueous stream to form a cation hydroxide base, PA1 (b) flowing a liquid sample stream containing anions to be detected and said eluent through a chromatographic separator portion of said first bed section, further comprising anion exchange material, to separate said anions to be detected, PA1 (c) flowing said aqueous separated anion stream through a second bed section substantially free of anion exchange material and including exchangeable hydronium ions, while applying an electrical potential to an anode in the said second bed to generate hydronium ions near said anode, to convert said base to weakly ionized form, and displacing some of said exchangeable hydronium ions with cations from said base, the cations electromigrating from the second bed section to said first bed section along a cation path in the cation exchange/material in the opposite direction to said aqueous feed stream to replenish exchangeable cations displaced from said first bed in step (a), and PA1 (d) flowing said suppressor effluent stream past a detector in which the separated anions in said suppressor effluent are detected. PA1 (a) a sample injection port PA1 (b) a flow-through bed of cation exchange material including exchangeable cations, the bed having first and second bed sections arranged in series, the first bed section being in fluid communication with said sample injection port, said first bed section further comprising a chromatographic separator portion including anion exchange material capable of separating anions in an aqueous sample stream flowing through said chromatographic separator portion, the second bed portion being substantially free of anion exchange material and being capable of converting base following in an aqueous stream therethrough into weakly ionized form, PA1 (c) first and second electrodes in electrical communication with said first and second bed sections, respectively, the cation exchange material in said first and second bed sections forming a cation path through said cation exchange material between said first and second electrodes, and PA1 (d) a power supply for applying a potential between said first and second electrodes.
The copending application describes apparatus for anion analysis including the following components:
Since hydrogen and oxygen gases are generated in the ion exchange bed which could interfere with detection, the copending application describes pressurizing the chromatographic effluent prior to detection, such as by use of a flow restrictor. The copending application also describes cation analysis by appropriate reversals of the cation and anion functional components.